Hello.
Is it possible to use IPA with HP-UX servers (ldapux) to authenticate
users from AD via IPA-AD trusts, or such way only work for systems with
sssd?

I suspect you need to test it -- set it up like against Netscape/iPlanet
directory server and use 'ipa-advise' recipes for FreeBSD or generic
Linux versions to get proper base DNs/attributes/objectclasses.